TS1365 What does it mean when my Macbook pro displays a question mark at startup, and won't boot?

My MacBook Pro won't start up. It displays a Question Mark for a while, and you can hear the HDD spinning, as well as some clicks, but nothing happens. It started after I accepted and began a recommended upgrade download and install. I started the upgrade install, rebooted, and walked away. An hour later, it displayed the progress bar as about 50% complete for the install. 24 hrs later, it was still at the same status. Several hours later, still no progress, so I hit the on/off button, shutting down the computer. After that, it would not restart...it gives the start-up chime, I hear the HDD spinning, and also hear some ominous "click click" sounds. Then a question mark pops onto the screen, and that's all. Any suggestions?

It means that it cannot find certain startup files. In your case it's probably because you interrupted the installation or the installation failed and files were corrupted or lost. Do the following:

Reinstalling Lion/Mountain Lion Without Erasing the Drive

Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.

When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu.

Symptoms

If you see a flashing question mark when you start your Mac, it's probably because it can't find the system software it needs to start up. Usually, all you have to do to get your Mac back up and running is remind it where its software is.

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